Ceramic materials are currently being used successfully in a variety of applications, such as for break rings in horizontal continuous casting processes. The horizontal continuous casting process involves extreme environmental conditions such as rapid rises in temperature, and severe temperature gradients. Generally, break rings for use in this type of application would be subjected to extremely fast temperature rises, and high temperature gradients often in excess of 1000.degree. C./cm. These conditions require a material that has good thermal shock resistance to prevent breaking. Additionally, in this type of application, the material should preferably have a high abrasive resistance and corrosion resistance with respect to molten metals, be machinable, and be economical to manufacture.
Boron nitride (BN) is presently being successfully used as a material for break rings due to its good thermal shock resistance, corrosion resistance, stability at high temperature, and machinability. However, it lacks good abrasion resistance, which renders it subject to high wear rates when exposed to flowing metal. Additionally, boron nitride ceramics typically contain a B.sub.2 O.sub.3 binder phase that can react chemically with molten metals, which further degrades the integrity of the boron nitride ceramic. The degradation of the boron nitride can also cause problems with the metal being cast. Boron nitride particles, as well as bubbles which form from gaseous B.sub.2 O.sub.3 or CO.sub.2 from the reaction of B.sub.2 O.sub.3 with carbon, can be trapped in the metal as it solidifies.
Alumina (A1.sub.2 O.sub.3) is also used in molten metal applications due to its hardness, abrasion resistance, and chemical stability. Although satisfactory, alumina ceramics often have poor thermal shock properties, and are difficult to machine because of their hardness. Thus ceramic components have been made with boron nitride and alumina in which the material has the abrasion resistance and chemical stability of the alumina and has the thermal shock resistance and good machinability of the boron nitride.
U.S. Pat. No. 4,007,049 discloses a thermal shock resistant material that has a high degree of resistance to failure by thermal fracture and which comprises a composite of a refractory oxide and flaked boron nitride. The boron nitride flakes are incorporated into a refractory oxide matrix as an inert, nonreactive, uniform dispersed phase in proportions sufficient to provide the oxide composite with an increased resistance to thermal shock.
British patent No. 784,704 discloses articles comprising boron nitride and refractory oxide, such as mullite in which the preferred compositions contain only up to 30 percent by weight boron nitride.
S. G. Tresvyatskii et al in "Effect of Boron Nitride Addition on Some Properties of Aluminosilicate Refractories", Institute for Materials Science Research, Academy of Sciences of the Ukrainian SSR, No. 4, pp. 36-39, April, 1968 discloses that the thermal shock resistance of aluminosilicate refractories can be increased with an addition of boron nitride.
Lewis et al in "Microstructure and Thermomechanical Properties in Alumina- and Mullite-Boron-Nitride Particulate Ceramic-Ceramic Composites", Ceram. Eng. Sci. Proc., 2:719-727 (Nos. 7-8, 1981) discloses the preparation of A1.sub.2 O.sub.3 -BN and mullite-BN composites and provides data on the thermal shock resistance of such composites.
U.S. Pat. No. 4,904,623 discloses a ceramic composite composed of a fused zirconia mullite with boron nitride that is recited as having good erosion and corrosion resistance to molten metals and good resistance to thermal shock. However, these composites have been found to expand up to 1100.degree. C. and then go through a shrinkage phase transformation of the zirconia mullite. In some applications, this phenomenon would lead to cracking due to the stress imparted to the composite during heating.
It is an object of the present invention to effectively correct for shrinkage at high temperatures by providing a ceramic material that has good erosion/corrosion resistance in high temperature environments.
Another object of the present invention is to provide a ceramic composite that is suitable for use as a break ring in a horizontal continuous casting process.
The above and further objects and advantages of this invention will become apparent upon consideration of the following detailed description thereof.